Monday, 2 February 2009

Evolving extruder

I have redesigned the lower half to be a lot simpler. I also wanted to see what would happen if I made a cavity of molten plastic inside the heater. Up till now I have been trying to minimise the amount of molten plastic to reduce ooze, but according to Anon's comments here, professional machines have a relatively large melt chamber. I wondered if plunging the filament into a chamber of already molten plastic would make it any easier to feed.

This is a cross section of my design: -

The plastic clamp and cylindrical finned heatsink have been replaced by a single horizontal 6mm thick aluminium plate that combines both roles.

The easiest and most accurate way to have made this would have been to mill it with HydraRaptor. If I make another, that will be the way I do it, but I made this one with a hack saw, a file and a drill press. I start by gluing a paper template on the aluminium with stencil mount.I then centre punch through the cross hairs and drill all the holes. The paper can be removed easily by dissolving the spray mount with paraffin. The larger mounting holes fit the Darwin X-carriage and the smaller ones fit HydraRaptor. The group of four holes allow the standard filament guide to be attached. The Darwin extruder clamp has slots but slightly oversized holes are fine.

The 8mm counter bore was a bit tricky. I drilled it with an 8mm drill and then bottomed it with an 8mm end mill. That showed that my drill press / mill is not really stiff enough to mill aluminium with an 8mm bit even though it has a 45mm thick steel pillar. I don't think HydraRaptor would have any problem doing it slowly with a small end mill. It probably doesn't make much difference if the counter bore does not have a flat bottom, so simply drilling would suffice.

Moving down the design is the PEEK insulator that forms the short thermal transition zone.

This is 8mm PEEK rod tapped with an M8 x 1 thread so it can screw into the heater block. I used the metric fine pitch because I didn't have the correct tap drill for M8 x 1.25 (6.75mm). The 3.5mm hole down the middle is drilled in-situ to ensure it lines up with the hole through the heatsink.

Small diameter PEEK rods are far more reasonably priced: 250mm x 8mm is only £3 here and is enough to make about 20.

The heater is a block of aluminium with a 6.5mm hole through it to take a vitreous enamel resistor for the heating element as described before.

As well as the tapped entrance to the melt chamber there is a small hole to take the thermistor.

I made this on my lathe, using a four jaw chuck. It could however be made with a drill press if the nozzle screwed into it instead of it screwing into the nozzle. The lathe gets all the faces perfectly square but there is no reason why it has to be accurate.

The next part down is a PEEK collar to insulate the heater from its retaining washer.

This is the only part I haven't thought of a way to make without a lathe. It might be possible to mill it with the right shaped cutter.

It snaps into the stainless steel washer and is a tight fit to the M6 spout so it anchors the nozzle laterally as well as vertically. It is counter bored at the back to reduce the thermal coupling.

Here is the assembly: -

It leaked a little bit of ABS but it seemed to stop when the leaked ABS oxidised. I should have sealed the joint with PTFE plumbers tape as I normally do. Apart from that it seems to work very well. I was able to manually push ABS through a 0.5mm nozzle very easily, at great speed. HDPE extrudes pretty quickly as well. When I stop pushing, it stops pretty quick. I think with a reversible drive ooze should be OK.

The design is much shorter than the previous one which will increase the build volume on Darwin. It is also very rigid so will not deflect when extruding.

I intend to simplify construction further. Rather than drill the stainless steel washer I can use the technique Ian Adkins uses on the BfB extruder where it is trapped between nuts and washers on studding. The only reason I did it this way was because I had the stainless steel bolts but did not have any M3 stainless studding.

I will also look at screw in nozzles. Andy Hall uses copper welding tips. The exit hole is a bit on the large size but I can reduce it by blocking it with high temperature solder and then drilling that, as I did with the solder sucker bit I tried.

19 comments:

Great this design looks easy to duplicate in a small home workshop. I like the idea of the plate thinking old Vero frame end plates.What is the distance between centers of the holes in the aluminum block?Maybe re-useing the original brass extruder barel cleaned up, cut to length solderd and drilled?The extruder has been the most dificult part to make this makes it much easier thank you.

I was always waiting for a _reliable_ printing head design to buy. (Im bugfixing pida instead of developing my repstrap).

But there is no reason to wait anymore, this design really looks simple. There are few bits which are unclear (but I think it is easy to understand when I begin to *build*, and not just looking at the pictures)

It's difficult to guess at the dimensions of the PEEK collar, but as an alternative, could you not wrap a dozen layers of PTFE tape around the barrel and then use a flat PEEK washer behind it.. the stresses horizontally on the PTFE I wouldn't expect to distort it much, most force is directed vertically with the washer behind under compression.

I think air is lousy heat transfer method to melt the filament and trying to melt it from surface contact on the tip as you insert it; is a dicey proposition and would take quite a bit of time at the right temperature.

In my case I had feed movement and assumed that it was melting at the face, when it actually was melting at the feed tube junction from heat transfer into the tube.

I guess the real question is how far and at X temperature will the melting travel? I image the experiment would be a solid block that is heated to say 240C at a contact point and quickly scooping out the melt zone.

Our previous barrels are nothing but surface contact once it enters the heat zone with a maximum depth of one half the fiber width.

I don't understand how yours jammed in mid air though. Admittedly I first primed mine by heating the block and then pushing the filament in. It melted as it hit the bottom and then filled the chamber. After that I always started with a chamber of molten plastic.

If I had first heated it with the filament in an empty chamber I don't think it would have jammed. Even if it formed a big lump in mid air there is nothing stopping me push the filament until it hits the bottom. The inlet is straight and smooth so solid plastic can slide down it. Once past the PEEK it is either in air or touching the metal. If in air it could cool but then I could just push it till it hit metal which would melt it again.

Once full the melting point only travels up the filament a few mm because the PEEK is cold at the top end. It forms a dynamic pressure seal. I.e. molten plastic flows up the gap between the cold filament and the PEEK but becomes solid plugging the leak. It then acts as a piston pressurising the melt, forcing it out the nozzle. As new filament is fed the front of the piston is melted away but the back is reinforced by new plastic reaching its glass transition and then swelling to seal the tube.

Was there something cold in the path of your filament after it had melted and swelled? What stopped the big lump being pushed to the bottom where it would have melted again?

First off operator error, I heated it up and then inserted the fiber and pulled it back so it wasn’t in contact with the face on the assumption most of my heating was by heat transfer via the tube walls. Then because of the geometry of the fitting and the heat transfer through it and the friction fit to the tube; it cross some sort of threshold and locked into place.

As you are threading the Peek have you considered threading both ends and just using that to hold the nozzle? It would reduce heat transfer paths. Or is the Peek to weak and/or unstable at extruding temperatures?

nophead, regarding the "next task" - I think you might want to talk to zac smith regarding the pinch-wheel design. it's seems to be gaining traction (pun intended), and it's perfectly designed to integrate with your revised heater head. It uses the washer-and-plate-separated-by-three-bolts design already, he just has the transitional zone between and the hot zone below the washer, where as you've squeezed them both into the smaller space